It's hard to categorize exactly what I like, because I'm partial to many things.
Generally, I'm inclined to dig a girl with some substance physically. I appreciate when a chick takes care of herself, and is healthy. I guess that's the politically correct answer, but that's like asking if I like blondes or brunettes--it's hard sometimes to define attraction.
"Thin" is alright I guess, so long as a girl doesn't starve herself/purge in order to get there--if you're naturally thin, then so be it. Legs have always been the thing for me though--and athletic is always sexy.
And ladies, don't be ashamed of your curves. 'Nuff said.